I can't answer specifically, Don, but I know that one of the long-term issues with the Speed Demon has been the drive line not living through the impact of each shift - just too much torque/torque change all at once, even with the MoTeC programmed to try to soften the hit each shirt. So - one reason for the seven-speed (eight, if you count the push truck) is to be able to run closer gears so there's not so much difference 'tween each. Yes, I assume there's a trade-off - what with the now being one extra shift per se to worry about hurting things.
Also -- maybe the 7-speed allows a slightly longer final drive ratio so the team can hit the goal of 500 mph without bogging too much in lower gears.